Satellite navigation systems such as the Global Positioning System (GPS), Glonass, Galileo, and BeiDou are used to provide position, velocity and timing information to a diverse range of applications. Satellite navigation systems work by broadcasting timing signals to users equipped with receiver devices.
Conventional receiver devices include a module that internally generates signals matching those broadcast by the satellites, referenced to an internal clock. These signals are in a defined format and are specified in open literature for each system. The receiver correlates these internal ‘replica’ signals with the received satellite signals to extract the correct signal timing and carrier phase and frequency for each satellite. The receiver also decodes the information contained within the satellite broadcast message that enables the receiver to know the location of each satellite.
The receiver uses the results of the correlation process to determine the time of flight for the received signals from each satellite. The time of flight is combined with the information of the location of several satellites in a mathematical process to calculate the position of the receiver antenna.
In addition to supporting an expanding range of commercial applications, satellite navigation systems are used in the field of security and defence. Whereas in the commercial domain, unencrypted, or Open Service, signals are used to obtain the position of the receiver, within the security and defence domain, secure encrypted signals are used. In the Galileo system, this is known as the Public Regulated Service, or PRS, and other systems provide similar services. The secure signals are also broadcast in a predefined format; however such formats are not public information. The satellite system is responsible for encrypting the signals, and the signals are decoded by a specialised secure receiver device.
In a known secure receiver, this is achieved through the use of a security module or security function. The security module is usually a physical component of the receiver. The security module/function contains and processes the cryptographic algorithms that enable the secure signal to be used by the receiver. The security module is capable of generating the required information to receive and process the encrypted signal as well as decoding the encrypted broadcast message that contains information on satellite orbits and clocks and other system and security information. Each receiver requires its own security module/function.
Such security modules will only operate if they have valid cryptographic keys and other parameters/information which are required to unlock the process of decoding the encrypted signals. The keys need to be loaded into the receiver. This process can be done manually (using specific equipment) or autonomously using techniques known as ‘over-the-air’. The keys and cryptographic material are critical to the security of the satellite navigation system and therefore there is a need to protect the distribution and management of the material. This can impose a large overhead on the operation of receivers that use encrypted navigation signals.
WO/2012/007720, in the name of Thales, describes a user receiver which includes a capability to capture samples of wideband signals including the Galileo Open Service and the encrypted Public Regulated Service (PRS). The user receiver includes a processing module that can process Open Service signals which are used to compute the position, velocity and time (also called ‘PVT’ information) of the user as described above. The Open Service PVT information is transmitted with the captured wideband samples to a central server. The central server has access to the secure PRS information which it uses to process the PRS elements of the captured signal. The server uses the PRS information to separately compute the PVT of the user. In this way, the number of PRS security modules which hold the secure cryptographic information can be reduced as it is not a requirement to include a security module in each receiver. The central server compares the Open Service PVT with the server computed PRS PVT and uses the information to carry out an authentication process.
However, this method relies on communicating large volumes of wideband RF captured signal. This has limitations in terms of available communications bandwidth, power required to broadcast the PRS signals samples and the cost of communications. In addition, it requires an Open Service receiver to operate at the user side. In many instances, the user does not require knowledge of his/her position (for example this location information may be provided to a third party, and used as an emergency beacon, or to provide information of how a vehicle is being driven, or the like). To operate an Open Service receiver unnecessarily places additional load on the battery at the user side. Further, the methods and apparatus described rely on the broadcast of open service PVT, and this signal can be intercepted, interfered with or deliberately jammed (which is one of the reasons for the existence of separate encrypted signals).
The NAVSYS Corporation have proposed a low cost GPS sensor, known as the TIDGET, described in U.S. Pat. No. 5,379,224, which utilises a distributed GPS software receiver concept. The TIDGET sensor does not track the GPS signals, but instead captures a “snapshot” of raw GPS samples which are communicated to the central server. At the server, the samples are processed to determine the user's PVT information. A related paper entitled ‘PPS Positioning in Weak Signal GPS Environments using a TIDGET Sensor’ (Proceedings of ION GNSS 2010, Portland, Oreg., September 2010) describes how the TIDGET concept could be applied to GPS military signals by using the field unit to capture the raw Radio Frequency (RF) data which is communicated to a central server for direct injection into a conventional GPS Precise Positioning Service (PPS) hardware receiver.